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Limit to number of photons in a single point

  1. Aug 17, 2010 #1
    Thought experiment:
    An X number of lasers shoot a photon simultaneously at a single point in space. Given constructive interference is there an energy limit that can be reached?

    At a single point is there a limit to the number of photons that can exist?
  2. jcsd
  3. Aug 17, 2010 #2
    IMO, above a certain density, the probability that they spontaneously become e+ e- pairs increases.
  4. Aug 19, 2010 #3
    Since quantum optics states that Photons cannot be seen in Phase. There would be no laws to prevent an infinite amount of photons at a single point. Because the photon is a Boson, they can occupy the same point in space.
  5. Aug 19, 2010 #4


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    There was a paper at PRL on this topic this week:
    "Limitations on the Attainable Intensity of High Power Lasers" by Fedotov et al. (Phys. Rev. Lett. 105, 080402 (2010))

    Basically they claim, that at some point, even the creation of a single e+ e- pair will be enough to start a cascade, which ends up in depleting the laser pulse. However, you need very large fields to enable pair creation from vacuum. There are some experiments in the next few years, which might test the hypothesis presented here, but up to now, it has not been verified.
  6. Aug 19, 2010 #5
    A photon will not interact with a photon. If a photon was to pass through another photon, there will be no change to there state. Photons are Bosons; they can occupy the same point in space. The experiment will not create electrons or electron neutrinos, there is no variation of coherent states or angles you can apply to the lasers which will create a particle with mass. im sorry to burst the bubble.
  7. Aug 19, 2010 #6
    Classically (at tree level) this is true. But a photon can couple to other photons in quantum mechanics, indirectly, through (for instance) electron-positron loops. This can also be treated classically via the introduction of non-linear terms in Maxwell's equations.

    There are indeed direct light-by-light scattering experiments which are under construction now. But already at lepton collider, the same has been investigated in another language : one probes a single real photon with a virtual photon. If you will, this is a re-interpretation of the above quantum corrections to the photon propagator as a "photon content" or structure function :
    Experimental Review of Photon Structure Function Data
    (and references therein)
  8. Aug 19, 2010 #7


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    So? Why don't you write a rebuttal to the peer-reviewed paper I cited earlier on? And while you are at it, you might also like to write rebuttals on:

    A. R. Bell et al.: "Possibility of Prolific Pair Production with High-Power Lasers", Phys. Rev. Lett. 101, 200403 (2008)
    C. K. Dumlu: "Schwinger vacuum pair production in chirped laser pulses", Phys. Rev. D 82, 045007 (2010)
    A. Ringwald: "Pair production from vacuum at the focus of an X-ray free electron laser", Physics Letters B 510, 107 (2001)
    E. Lundström et al.: "Using high-power lasers for detection of elastic photon-photon scattering", Phys. Rev. Lett. 96, 083602 (2006)
    and of course to start at the very beginning
    G. Breit and J. A. Wheeler: "Collision of Two Light Quanta", Phys. Rev. 46, 1087–1091 (1934).

    And as you stated that photons do not have momentum in another thread (https://www.physicsforums.com/showthread.php?t=420446"), you might also want to write rebuttals on any paper mentioning Compton scattering.

    Or do you already have any peer-reviewed publications at hand backing up your claim?
    Last edited by a moderator: Apr 25, 2017
  9. Aug 21, 2010 #8
    I don’t need a physics paper to know photons are bosons. The production of particles are due to other present variables in the experiment. The question was asking if there is a limit to the amount of photons at a single point. The answer is no. If you want to start talking about polaritons or cavity photons to exercise your vocabulary, my ears are open. There is not a paper that would have been passed by the education system that would show the limit of photons at a single point.
    Last edited by a moderator: Apr 25, 2017
  10. Aug 21, 2010 #9

    Vanadium 50

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    There are two separate questions here.

    One is "is there a limit on energy density" and the answer is "yes". This is often called "vacuum sparking" in the literature.

    The other is "is there a limit on photon number density" which is more complicated. I don't think the number operator commutes with the position operator, which would imply the question itself is not well defined.
  11. Aug 23, 2010 #10


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    Well, the other thing present is the vacuum. Nothing else. However, I guess ther is no possibility to remove the vacuum from any experiment, so one should consider interactions of a light field with the vacuum as intrinsic.

    The first question was about energy densities. However even the answer to the question about the highest possible number of photons in a single point (or small region) is not as easy as you would like it to be. Also Helium 4, Carbon 12 and (as you brought up the point) exciton polaritons are bosons. Nevertheless, there is a highest possible number of those bosons inside one small regions because they are composite bosons with inner structure and fermionic constituents. At high enough density, exciton polaritons will undergo a Mott transition away from a condensed state, for example.

    For photons this means that it really matters whether photons are elementary bosons at any field strength or whether at some point, there is some hadronic or leptonic contribution at large fields. There are lots of papers out there reporting on the actual state of experimental and theoretical results regarding the photon structure function and Schwinger pair production from the vacuum. While I agree that photons are perfect bosons at commonly experienced densities or in your common laser lab, it is not sensible to extrapolate that intuition to large densities under which the interaction with the vacuum state cannot be neglected anymore.
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